Lasers are well known devices employed as a light source, and have been shown to emit light throughout the visible spectrum at very narrow linewidths. Tunable dye lasers are well known and are widely applied in many fields.
Such tunable lasers employ means to “tune” the laser to emit light at differing wavelengths. Such tunable dye lasers can use a liquid gain media, such as disclosed in U.S. Pat. No. 5,181,222 (Duarte), commonly assigned and incorporated herein by reference. FIG. 1 shows a schematic diagram of a dye laser apparatus 20 disclosed in U.S. Pat. No. 5,181,222. As illustrated, apparatus 20 includes a narrow linewidth laser output beam indicated at 22 by parallel dashed lines. The diameter of beam 22 is indicated at W. A dye cell 24 (which can be like the one described in U.S. Pat. No. 4,891,817) is “pumped” or excited by a beam 26 from a source such as a copper laser. This phenomenon is well known in the art. The pulse repetition frequency (prf) of such a source is in the range from 5 kHz to 20 kHz. Other laser pump sources, such as the N2 laser, deliver pulse repetition frequencies in the 1 Hz-100 Hz range. Forming part of an optical cavity of the laser apparatus 20 is a first prism 30 which receives laser emission from dye cell 24 at an incident angle indicated at φ1,1. Laser light (indicated by the shaded area) from prism 30 is directed at an angle φ1,2 onto a second prism 32 and thence is refracted at an angle ψ1,2 in an expanded beam (shaded area) onto a Littrow-mounted grating 34. The angle of light incident on and diffracted from grating 34 is indicated by an angle θ. The relationships of these angles to the laser beams within the multiple-prism Littrow-mounted grating (MPL) portions of the optical cavity are given in detail in a book entitled DYE LASER PRINCIPLES by Duarte, an inventor of the present invention. After being diffracted back from Littrow-mounted grating 34, through prism 32 and prism 30, the laser light is highly polarized and frequency narrowed. The plane of polarization here lies parallel to the plane of FIG. 1. This polarized light passes to the left back through dye cell 24 for further amplification and becomes a narrow linewidth laser beam 38 having the diameter W. As the polarized beam 38 continues to the left from dye cell 24, it encounters a specially provided, partially reflecting polarizer device 40. The outer or left-most face of polarizer device 40 is made partially reflecting by a suitable coating 42, such as a very thin layer of low-loss dielectric material, which gives about 5% to 20% reflection of laser beam 38. The remaining 80% to 95% of beam 38 passes through reflecting coating 42 and becomes laser output beam 22. Laser beam 22 is polarized in a plane parallel to the plane of FIG. 1.
There are several disadvantages to such dye lasers. The lasers can employ ethanol and methanol solvents, and therefore can be a safety concern. Further, such dye lasers are not compact, and compactness is needed in some applications.
Solid state dye lasers were introduced in the 1960s. (Refer to Soffer and McFarland, CONTINUOUSLY TUNABLE NARROW-BAND ORGANIC DYE LASER, Appl. Phys. Lett. 10, 266-267 (1967), and Peterson and Snavely, STIMULATED EMISSION FROM FLASHLAMP-EXCITED ORGANIC DYES IN POLYMETHYL METHACRYLATE, Appl. Phy. Lett. 12, 238-240, (1968).) The gain media employed by these early solid state dye lasers was dye-doped PMMA (polymethyl methacrylate). The early PMMA was plagued by inhomogenities which severely limited their scope. As such, developments in this area were not pursued until the early 1990s when researchers developed a highly homogeneous from of dye-doped PMMA, which they referred to as MPMMA (modified polymethyl methacrylate). (Refer to Maslyukov et al, SOLID-STATE DYE LASER WITH MODIFIED POLY(METHYL METHACRYLATE)-DOPED ACTIVE ELEMENTS, Appl. Opt. 34, 1516-1518, 1995.)
A very narrow linewidth tunable emission laser was developed using the dye-doped MPMMA in the mid-1990s by one of the inventors of the present invention. (Refer to Duarte, SOLID-STATE MULTIPLE-PRISM GRATING DYE LASER OSCILATORS, Appl. Opt. 33, 3857-3869 (1994), and Duarte, SOLID-STATE DISPERSIVE DYE LASER OSCILLATOR: VERY COMPACT CAVITY, Opt. Commun. 117, 480-484 (1995).) However, a disadvantage of the MPMMA and other types of dye-doped polymers, such as HEMA:MMA (referred to in Costela et al, POLYMERIC MATRICES FOR LASING DYES: RECENT DEVELOPMENTS, Laser Chem. 18, 63-84.), is a high dn/dT value, wherein n is the refractive index and T is the temperature. In pure dye-doped polymer matrices, dn/dT is in the range of about −1.2×10−4 to about −1.4×10−4(degree-K)−1. A high dn/dT value prevents operation of the laser at high repetition rates and higher pulsed energies. For instance, the use of high repetition rates causes increased beam divergence and other undesirable features such as a loss of spectral coherence.
In the late 1980s and early 1990s, sol gels and other polymer silicate composites were introduced for addressing thermal issues. (Refer to Duarte et al, A NEW TUNABLE DYE LASER OSCILLATOR: PRELIMARY REPORT, in Proceedings of the International Conference on Lasers '92, Wang, Ed. (STS, McLean, Va., 1993) pp 293-296, and Salin et al, Opt. Lett. 14, 785 (1989), and Altman et al, IEEE Photon. Technol. Lett. 3, 189 (1991).) Unfortunately, these sol gels exhibited internal optical inhomogenities that manifested via severe laser beam distortions, as shown in FIG. 2.
Accordingly, a need exists for a gain media that exhibits optical homogeneity and has a low dn/dT value.
The present invention provides an optically homogeneous gain media material. The material is easy to manufacture, low in cost, can be polished to form facets, maintain stable performance characteristics over a broad temperature range, and provides for reduced laser beam divergence.